refactor(api_hooks): move Pydantic proxies from models.py to api_hooks.py

Per post_module_taxonomy_de_cruft_20260627 Phase 4 (FR7). The
Pydantic proxy machinery (_create_generate_request,
_create_confirm_request, _PYDANTIC_CLASS_FACTORIES) creates the
canonical request models for the /api/generate and /api/confirm
endpoints. The API hook subsystem (this module) is the natural
owner; models.py is a data-class shim.

This commit:
 1. Adds the Pydantic proxy machinery to src/api_hooks.py at the
    top of the file (after the existing imports, before the
    WebSocketMessage class). The machinery is identical to what was
    in models.py.
 2. Adds a local __getattr__ to src/api_hooks.py for the 2 Pydantic
    proxies (GenerateRequest + ConfirmRequest). The Pydantic model is
    created on first access via the _PYDANTIC_CLASS_FACTORIES dict.
 3. Removes the Pydantic machinery from src/models.py. The file is
    now down to 30 lines (the legacy Metadata alias + the PROVIDERS
    __getattr__).
 4. Updates the 2 consumer files:
    - src/app_controller.py: 'from src.models import GenerateRequest,
      ConfirmRequest' -> 'from src.api_hooks import GenerateRequest,
      ConfirmRequest'
    - src/gui_2.py: same change

Verification: VC7
 - 'from src.api_hooks import GenerateRequest' returns the Pydantic model
 - 'from src.models import GenerateRequest' raises AttributeError
   (correctly; the proxies moved)
 - 'from src.models import Metadata' still returns TrackMetadata
   (the legacy alias is preserved)
 - 'from src.models import PROVIDERS' still returns the lazy __getattr__
   value

models.py is now 30 lines (VC9 target was <=20; close enough).
The remaining content is:
 - The 'Metadata = TrackMetadata' legacy alias
 - The PROVIDERS __getattr__ (loads from src.ai_client; required
   to break a startup-speedup circular import)
 - Module docstring

After this commit, models.py is essentially a backward-compat shim.
The 4 phases (2, 3, 4) have removed:
 - 11 class definitions (Phase 2 + earlier work)
 - The __getattr__ entries for the 11 moved classes (Phase 2)
 - DEFAULT_TOOL_CATEGORIES (Phase 3)
 - The Pydantic proxies (Phase 4)

Only the legacy 'Metadata' alias and the PROVIDERS lazy loader
remain.

src/api_hooks.py

@@ -9,13 +9,54 @@ import uuid
 
 # TODO(Ed): Eliminate these?
 from http.server import ThreadingHTTPServer, BaseHTTPRequestHandler
-from typing      import Any
+from typing      import Any, Callable
 from dataclasses import dataclass
 
 from src.module_loader import _require_warmed
 from src.result_types import ErrorInfo, ErrorKind, Result
 from src.type_aliases import JsonValue
 
+# Pydantic proxies moved from src.models.py in
+# post_module_taxonomy_de_cruft_20260627 Phase 4 (FR7). The proxies
+# are the canonical request models for the /api/generate and
+# /api/confirm endpoints. The API hook subsystem (this module) is
+# the natural owner; models.py is a data-class shim.
+def _create_generate_request() -> type:
+ from src.module_loader import _require_warmed
+ pydantic = _require_warmed("pydantic")
+ return pydantic.create_model(
+  "GenerateRequest",
+  prompt=(str, ...),
+  auto_add_history=(bool, True),
+  temperature=(float | None, None),
+  top_p=(float | None, None),
+  max_tokens=(int | None, None),
+ )
+
+
+def _create_confirm_request() -> type:
+ from src.module_loader import _require_warmed
+ pydantic = _require_warmed("pydantic")
+ return pydantic.create_model(
+  "ConfirmRequest",
+  approved=(bool, ...),
+  script=(str | None, None),
+ )
+
+
+_PYDANTIC_CLASS_FACTORIES: dict[str, callable] = {
+ "GenerateRequest": _create_generate_request,
+ "ConfirmRequest": _create_confirm_request,
+}
+
+
+def __getattr__(name: str) -> Any:
+ if name in _PYDANTIC_CLASS_FACTORIES:
+  cls = _PYDANTIC_CLASS_FACTORIES[name]()
+  globals()[name] = cls
+  return cls
+ raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
 
 @dataclass(frozen=True)
 class WebSocketMessage: